Heat treating bath



Patented May 21, 1940 PATENT OFFICE HEAT TREATING BATH Carl Albrecht, Kronberg, in Taunus, Germany, assignor to Deutsche Gold und Silber Scheid eanstalt, vormals Roessler, Frankfort-on-the- Main, Germany, a corporation of Germany No Drawing. Application November 16, 1937, Serial No. 174,841. In Germany December 30,

1 Claim. (Cl. 14813.1)

. This invention relates to fused salt baths used for heat treating metals and more particularly to the heat treatment of light metals in such baths.

It has been known heretofore to heat treat var- 5 ious metals in fused salt baths for the purpose of annealing,'hardening and otherwise, by heat treatment, causing desired changes in the structure of the metal. This method of heat treatment is frequently used to heat'treat various light metals, for example, aluminum and aluminum alloys. For the heat treatment of. such light metals relatively low melting fused salts are commonly used, for example, baths made by fusing alkali metal nitrates or nitrites or mixtures thereof. Such baths are commonly made by fusing together different mixtures of potassium and sodium nitrates and nitrites, with or without other salts, the particular composition depending upon the melting point desired.

In the operationof salt baths for heat treating metals, corrosion of the metal being treated or of. the metal container in which the fused bath is maintained frequently occurs. This corrosive effect frequently occurs after the bath has been in operation for some time. Aluminum, aluminum alloys and similar light metals are especially susceptible to this corrosive'effect.

An object of the present invention is to provide an improved method for operating fused salt baths, especially for the heat treatment of light metals. The invention likewise comprises an improved fused salt bath for heat treating various metals. A further object is to provide a simple and effective method for inhibiting or preventing the corrosion of metals in contact with a fused salt heat treating bath and more especially to prevent the corrosion of light metals such as the aluminiferous metals -when in contact with a fused salt bath. Another object is to prevent the formation of corrosive substances in fused salt baths containing decomposable alkali metal compounds such as the nitrates and nitrites. Other objects will be apparent from the following description of the invention.

The present applicant-has discovered that the corrosive effects of fused salt baths which have been operated for some time are due to the presence in the bath of alkaline substances, for example, alkali metal oxides and carbonates. Such alkaline substances may exist as impurities in the salts originally fused to make the heat treating bath but more commonly are formed in the heat treating bath by thermal or other decomposition of the salt components. The corrosive effects of heat treating baths are more commonly experisalts.

enced in baths containing alkali metal nitrates and nitrites and it has been found that in these baths the nitrates and nitrites tend to decompose to form alkaline substances, e. g., alkali metal oxides or carbonates. It has been ascertained that the presence of even less than 1% of such alkaline substances in the heat treating bath may cause injurious corrosion to light metals in contact with the bath.

Applicant has further discovered that the above described corrosive effects of salt baths may be prevented by adding to or incorporating in the fused salt mixture inorganic acidic materials which react with the alkaline materials 'under the temperature conditions of the bath operation. Various acidic salts or other acid releasing materials may be used, for example, alkali metal dichromates, alkali metal metaphosphates, alkali metal pyrophosphates, and similar ammonium In addition to the acidic ammonium salts,

e. g., ammonium dichromate and the like, the

neutral ammonium salts of mineral acids, for example ammonium chloride or ammonium nitrate may likewise be used for the purpose of the present invention.

In practicing the present invention, a fused salt bath, for example, one containing alkali metal nitrates or nitrites or both is made up and operated in the usual manner. The presence of deleterious amounts of alkali materials in the heat treating bath may be determined either by in the calculated amount sufficient to react with the alkali metal oxide and carbonate present to convert the dichromate to neutral chromate. Various other suitable acidic materials may be similarly added to form neutral salts in place of the alkaline materials present in the bath. After the bath has been so treated it has' substantially no corrosive effect on metals and the bath may be used successfully for heat treating various light metals which are susceptible to corrosion by alkaline substances. It is preferable to add the acidic material continuously or intermittently during extended operation of the heat treating bath, so as to maintain the bath in a neutral and non-corrosive state. In continuous operation of a heat treating bath in commercial practice, it is often possible to ascertain the rate of formation of alkaline materials in the fused bath and, this having been determined, the bath may be maintained in a neutral non-corrosive state by the regular addition of the required amounts of the acidic material. If analysis shows that the ingredients of a fused bath prior to fusing contain alkaline materials, the proper amount of acidic material may be mixed with the ingredients prior to fusion so that the resulting fused bath will be non-corrosive. I

In one modification of the present invention the acidic material is intimately mixed with one or more of the bath ingredients or other suitable diluent and this mixture is added to the fused salt bath as required. The desired intimate mixture of the acidic material and bath ingredients may be made by various methods; for example, the materials may be fused together, a pulverulent mixture may be compressed to briquettes or a solution containing the constituents may be evaporated together to get a mixture of crystals. It is preferable in using such mixtures first to mold or compress them into briquettes or pellets to be added to the fused bath. This method of adding acidic material is advantageous and preferred for adding various acidic materials which may tend to react unfavorably with the fused bath constituents at the operating temperature of the heat treating bath. For example, when a dichromate is added to a fused bath containing alkali metal nitrite at the usual working temperatures, e. g., 400-550 C, the' dichromate tends to react with the nitrite, decomposing the nitrite and producing nitrous gases. This disadvantage is readily overcome by introducing the dichromate in the form-of an intimate mixture with one or more of the bath constituents or other diluent. When making such additions of. the acidic material, in admixture with the bath constituents or other suitable diluents, the quantity of the diluting material preferably is at least fivetimes the amount of the acidic material added."

for adverse reaction such as that which occurs 20 between dichromate and nitrite.

I claim:

In a process for heat treating metal in a fused salt bath containing a nitrogenous alkali metal salt, the step which comprises periodically adding 25 to said bath an inorganic substance of acidic nature in intimate admixture with a nitrogenous alkali metal salt constituent of said bath in an amount sufflcient to constantly maintain said bath substantially neutral whereby metal con- 30 tacted therewith is not corroded.

- CARL ALBRECHT. 

